The present invention relates generally to mechanical lift systems and, in particular, relates to a method and apparatus for fixing the position of a hydraulic cylinder in an extended position while avoiding the need to maintain high hydraulic pressure.
Referring to FIG. 1A, conventional lifting systems include one or more hydraulic actuators 20, each having a cylinder 22 that includes an annular side wall 41 closed at its lower end by a base 32, and closed at its upper end by an upper wall 40. Cylinder 22 houses a movable piston 26 that extends through an aperture 43 formed in upper wall 40. Piston 26 includes a piston head 28 that slides and seals against the inner surface of the annular wall 41 to define a hydraulic chamber 30 disposed between the base 32 of cylinder 22 and the piston head 28.
In single-acting actuators, high pressure fluid is delivered to chamber 30 via a port 34 that is connected to a fluid source (not shown). The pressure acts on the undersurface of the piston head 28 to bias the piston 26 and supported load (not shown) upwardly. Hydraulic fluid may be returned from the chamber 30 to a tank (not shown) via orifice 34 when it is desirable to retract the piston 26.
As the piston 26 is maintained in an extended position over time, hydraulic pressure delivered by the pressure source tends to wane, or fluid tends to leak, and the weight of the load begins to force the piston 26 from its extended position. It has thus become desirable to provide an external member that supports piston 26 once the piston 26 has reached its desired extension, thereby reducing or eliminating the need to deliver continued hydraulic pressure to the cylinder chamber 30.
Conventional systems therefore employ an annular locknut 46 having inner threads 48 that mate with outer threads 50 of cylinder 22. Threads 48 and 50 have a pitch such that rotation of locknut 46 translates the locknut up and down along piston 26. Accordingly, when the piston 26 is extended, the locknut 46 can be raised to a position whereby the upper surface of the locknut directly engages the lower surface of the load. The piston 26 may then be lowered with the load supported entirely by locknut 46. However, if small angular misalignments exist between the load and the upper surface of the locknut 46, the weight of the load will not be equally distributed along the entire upper surface of the locknut. Rather, a large amount of weight will be supported at a small portion of the locknut, for example an edge, thereby subjecting the undersurface of the load along with the edge of the locknut 46 to potential damage associated with the high forces.
Accordingly, referring now to FIG. 1B, conventional actuators 20 may employ a tilt saddle 49 having a domed lower surface that engages a ball-and-socket joint formed in the upper surface of piston 26. Actuator 20 further includes a locknut 46 defining a threaded cylindrical bore that mates with corresponding threads on the outer surface of piston 26 at a location between upper wall 40 and tilt saddle 49. Accordingly, locknut 46 may be raised and lowered by rotating the locknut with respect to the piston 26.
During operation, when the piston 26 is extended, the locknut 46 can be lowered against the upper wall 40 of the cylinder 22 to create an interference that prevents the piston 26 from being retracted even when the hydraulic fluid is returned from chamber 30 to tank. The locknut 46 is screwed upwardly along the piston 26 when the piston 26 is to be retracted. Once piston 26 is extended to engage the undersurface of a load, tilt saddle 49 wobbles to compensate for small angle misalignments between the load and the base 32 or foundation on which the actuator 20 is supported.
The tilt saddle/locknut combination has been suitable for use with actuators having single-acting cylinders, but not for double-acting cylinders, as the threaded piston rod prevents making a seal at the gland where it exits the cylinder. Double-acting cylinders are useful to permit power retraction of the piston.